By placing its hand upon a plate the highly trained macaque indicates its readiness to attempt detection of a signal which will occcur at random within 10-20 sec. If it removes its hand from the plate within 150-1000 msec after signal onset, it is presumed to have detected the signal and is rewarded. Random removals are punished with air puff and delay of next available attempt. Animals trained in this manner become highly reliable psychophysical observers and "report" in this fashion thresholds which remain constant over a period of many months for electrical excitation of striate cortex via permanently implanted electrodes. Against this background they are then tested for threshold to stimulation, 0.2-msec pulses, 50-100 Hz, with a microelectrode passed sequentially through the cortical laminae. Laminar position is monitored by background activity and photically evoked response. Two minima in threshold usually occur in such traverses. They are usually 2-3 fold less than at adjacent sites and are presently thought to be associated with the inner and outer bands of Baillarger. Transition from nondetection to detection can occur for stimulus changes less than or equal to 1.0 MuA. Reports from human subjects (Brindley and Lewin; Dobelle and Mladejovsky) emphasize the uniformity of the effect produced by electrical excitation any place in striate cortex, and data from macaques are concordant with this. Thus, an unusual opportunity is at hand: a sharp threshold transition consequent to a precisely controlled physical input at a fixed and known location in the nervous system, producing an immediately assessable behavioral consequence. By assaying the synaptic events with current source density analysis in the vicinity of the stimulating microelectrode, as well as the single unit activity engendered by detectable versus nondetectable pulses, it should be possible to arrive at a firm definition of the neural events initiating a conscious perception. Additionally, data will be acquired concerning interlaminar transactions and the neural response to interjected activity, facts of relevance to any prosthetic or therapeutic use of electrical stimulation of the central nervous system, and for understanding the organization of visual processes.